1. Field of the Invention
The present invention relates to DC-AC inverters. More specifically, it relates to DC-AC inverters that adapt to different input voltages and different loads.
2. Discussion of the Related Art
Producing a color image using a Liquid Crystal Display (LCD) is well known. Such displays are particularly useful for producing images that are updated by frames, such as in LCD desktop and laptop computer. Typically, each image frame is composed of color sub-frames, usually red, green and blue sub-frames.
LCD systems employ a light crystal light panel that is comprised of a large number of individual liquid crystal pixel elements. Those pixel elements are beneficially organized in a matrix comprised of pixel rows and pixel columns. To produce a desired image, the individual pixel elements are modulated in accordance with image information. Typically, the image information is applied to the individual pixel elements by rows, with each pixel row being addressed in each frame period.
Pixel element matrix arrays are preferably xe2x80x9cactivexe2x80x9d in that each pixel element is connected to an active switching element of a matrix of switching elements. One particularly useful active matrix liquid crystal display is produced on a silicon substrate. Thin film transistors (TFTs) are usually used as the active switching elements. Such LCD displays can support a high pixel density because the TFTs and their interconnections can be integrated on the silicon substrate.
FIG. 1 schematically illustrates a single pixel element 10 of a typical LCD. The pixel element 10 is comprised of a twisted nematic liquid crystal layer 12 that is disposed between a transparent common electrode 14 and a transparent pixel electrode 16. Additionally, image signals are applied to the pixel electrode 16 via a control terminal 24.
Still referring to FIG. 1, the liquid crystal layer 12 rotates the polarization of light 30 that passes through it, with the rotation being dependent on the voltage across the liquid crystal layer 12 (the image signal potential). The light 30 is derived from incident non-polarized light 32 from an external light source (which is not shown in FIG. 1). The non-polarized light is polarized by a first polarizer 34 to form the polarized light 30. The light 30 passes through the transparent pixel electrode 16, through the liquid crystal layer 12, and through the transparent common electrode 14. Then, the light 30 is directed onto a second polarizer 36. During the pass through the liquid crystal layer 12, the polarization of the light 30 is rotated in accord with the magnitude of the voltage across the liquid crystal layer 12 (the image signal potential). Only the portion of the light 30 that is parallel with the polarization direction of the second polarizer 36 passes through that polarizer. Since the passed portion depends on the amount of polarization rotation, which in turn depends on the voltage across the liquid crystal layer 12, the voltage on the control terminal 24 controls the intensity of the light that leaves the pixel element.
FIG. 2 schematically illustrates a liquid crystal display comprised of a pixel element matrix. As shown, a plurality of pixel elements 10, each having an associated switching thin film transistor, are arranged in a matrix of rows (horizontal) and columns (vertical). For simplicity, only a small portion of a pixel element matrix array is shown. In practice there are numerous rows, say 1290, and numerous columns, say 1024. Still referring to FIG. 2, the pixel elements of a row are selected by applying a gate (switch) control signal on a gate line, specifically the gate lines 40a, 40b, and 40c. Image signals are then applied to column lines 46a, 46b, and 46c. The various image signal voltages are then applied to associated control terminals 24 of the pixel elements 10. When the gate (switch) control signal is removed, the image signal voltages are then stored on capacitances associated with the TFT.
The foregoing processes are generally well known and are typically performed using digital shift registers, microcontrollers, and voltage sources. Beneficially semiconductor processing technology is used extensively.
The principles of the present invention relate to producing the non-polarized light 32 illustrated in FIG. 1. That non-polarized light 32 is typically produced by a cold cathode fluorescent lamp. This is at least partially because fluorescent lamps are efficient sources of broad-area white light. In battery powered applications, such as portable computers, the efficiency of the fluorescent lamp light source directly impacts battery life, size, and weight.
Fluorescent lamps are typically powered by an inverter. The inverter, in turn, can be powered by a battery or by another power source such as an LCD power supply. In any event, the inverter converts a relatively low DC voltage (say 3-24 volts DC) into a high AC voltage required to drive the fluorescent lamp. Typically over 500 volts are required to operate a cold cathode fluorescent lamp, while a xe2x80x9ckick-offxe2x80x9d voltage of around 1500 Volts is required to start conduction. Thus, such inverters are DC-to-AC inverters.
FIG. 3 depicts a conventional DC-to-AC inverter 50 in operation. That inverter receives DC power on a line 52. The operating DC-to-AC inverter includes a filter capacitor 54, totem pole arranged FET switches 56 and 58, diodes 57 and 59, an inductor 60, one or more fluorescent lamps (modeled by resistors) 62, each associated with a transformer 64, and a storage capacitor 66. The FET switches 56 and 58 are controlled by a controller 68. In operation, the FET switches 56 and 58 are alternately turned on and off with about equal times (50 % duty cycle) by the controller 68. When the FET 56 is conducting, the FET 58 is OFF. Then, the input on line 52 is switched across the inductor 60 and transformer(s) 64 and the storage capacitance 66. When FET 56 is OFF, the FET 58 is conducting. Additionally, under proper bias conditions, the diodes 57 and 59 conduct. Then, the storage capacitor 66 discharges through the inductor 60 and the transformer(s) 64 to ground.
Essentially, the DC-to-AC inverter 50 forms a simplified circuit shown in FIG. 4. The input voltage supply 80 is formed by the controller 68 selectively switching the FET switches 56 and 58 such that the power input on line 52 is applied to the inductor 60, and then selectively switching that inductor to ground. FIG. 4 also shows an equivalent inductor 84, which is formed by the inductance of the inductor 60 and of the transformer(s) 64. That equivalent inductor 84 beneficially resonates with an equivalent resonant capacitor 80, which is the reflected secondary-side capacitance of the lamp-shield capacitance and the inter-winding parasitic capacitance of the transformer. FIG. 4 also shows an equivalent resistor 90, which represents the transformed resistance of the fluorescent lamp(s) 62.
While DC-to-AC inverters as shown in FIGS. 3 and 4 are generally successful, in some applications they may not be optimal. For example, it is difficult to implement highly efficient DC-to-AC inverters over a wide range of input voltages. That is, the voltage on line 52 becomes critical in the overall design of the DC-to-AC inverters, and thus to the LCD display. In practice DC-to-AC inverters must be tailored to a particular LCD display""s backlight inverter input voltage.
Even if a DC-to-AC inverter""s input voltage range is acceptable, a DC-to-AC inverter usually only works well when designed for a particular load. That is, the equivalent lamp resistance 90 (see FIG. 4) and capacitance 80 must be taken into consideration when designing a particular DC-to-AC inverter. Thus, DC-to-AC inverters are usually designed to operate only with a narrow range of fluorescent lamps. Changes in lamp styles, sizes, or manufacturers can create problems.
The foregoing problems with DC-to-AC inverters mean that prior art LCD display DC-to-AC inverters either were designed for a particular application, or that inefficient operation had to be accepted. Since neither choice is desirable, a new DC-to-AC inverter that is adaptable to different input voltages and loads (fluorescent lamps) would be beneficial.
Accordingly, the principles of the present invention provide for systems, such as LCD displays, that include DC-to-AC inverters that are adaptable for use with different input voltages and different loads. In LCD displays, this enables different lamps to be operated under different input voltage conditions without requiring a new DC-to-AC inverter design. Such is particularly beneficial in reducing costs since a given DC-to-AC inverter design will work in many different applications, thus enabling economies of scale.
A DC-AC inverter that is according to the principles of the present invention includes a voltage-step-up network, with the step-up voltage set by a controller that drives totem-pole configured FET switches according to the desired step-up voltage. The controller beneficially regulates its duty cycle in response to current and/or voltage feedback signals. Also beneficially, the DC-AC inverter includes a configurable inductor and a configurable transformer. Such configurable components enable efficient operation with different loads. Such DC-AC inverters are particularly useful in driving liquid crystal display lamps. When the lamps are behind the LCD pixel array, the DC-to-AC inverter is often referred to as a backlight inverter.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.